The control of nuclear structure and function by the cell cytoplasm will be studied using as a model system the transformation of the sea urchin sperm nucleus after its introduction into the egg cytoplasm at fertilization. Initial studies will focus on the order of appearance of certain biochemical and structural alterations of the sperm chromatin. We will study quantitatively the loss of sperm specific histone and the acquisition by the male pronucleus of non-histone chromosomal proteins, embryonic histones and enzymes of replication and transcription. The dependence of these transitions on macromolecular synthesis and their relationships to the initiation of DNA and RNA synthesis, decondensation -condensation of the chromatin, stage of the cell cycle at the time of fertilization and mode of egg activation at fertilization will be documented. Our eventual aim will be to mimic some of these events in cell free preparations and to isolate the responsible components of egg cytoplasm.